1. Field of the Invention
This invention relates to voltage regulation circuitry, and more particularly to linear, or series, voltage regulation circuitry which provides an output signal that is activated when the voltage of an unregulated input voltage source may be insufficient for proper output voltage regulation.
2. Description of the Related Art
Voltage regulators are employed in nearly all electronics systems to convert available unregulated power to power at specified voltage levels. Series, or linear, voltage regulators employ a power transistor as a "pass" transistor between the unregulated input voltage and the regulated output voltage. As the input voltage and output current vary, feedback to the pass transistor, which is operated in the active region, causes the voltage drop across the transistor to compensate for these variations, and thus maintain the desired output voltage.
However, the pass transistor (typically a pnp transistor for low-dropout applications) requires a minimum voltage drop to maintain proper operation. If the input voltage drops to a level that does not provide this minimum voltage difference, or "headroom", across the pass transistor, the output voltage will fall out of regulation. If the voltage regulator could provide a signal, an alarm, which indicates that there is insufficient headroom to guarantee a properly regulated output voltage, the operational circuitry which relies upon the regulator for power could take measures to ensure that no data is lost, no in-correct commands are executed, that no logic states are mistakenly switched. In short, given such a warning, the operational circuitry could take action to prevent and correct errors and could also take steps to reduce power consumption. Unfortunately, headroom requirements vary widely from application to application.
Series regulators are available with alarm outputs which are triggered whenever the output voltage falls a given percentage out of regulation (e.g. the LP2951 series available from National Semiconductor, Corporation, Santa Clara, Calif. However, because the alarm is triggered when the output voltage is already out of regulation, there may be too little time for preventive and corrective measures. An alarm based upon the unregulated input voltage may provide greater warning to the system designer.
With additional circuitry and an extra pin on the voltage regulator's package, the regulator could accommodate various headroom requirements. That is, a designer employing the regulator could set the threshold input voltage at which the alarm signal is activated using a voltage divider and the anticipated requirements of a particular design. Naturally, the additional circuitry in the regulator, the extra pin on the regulator's package and the additional circuitry external to the regulator (i.e., its voltage divider) would increase the cost, volume and weight while reducing the reliability of any system employing such a design.
Portable, battery-powered systems are particularly vulnerable to the vagaries of voltage regulation. Over the course of discharging a battery its terminal voltage drops significantly. Portable system designers go to great lengths to extend the time a system may operate without requiring re-charging (or new batteries, if primary batteries are used). The designer could always add batteries to increase the system's nominal operating capacity, but the added expense and particularly the added weight limits the utility of this approach.
To accommodate the reduction in terminal voltage that accompanies a battery's discharge, system designers add cells to the batteries, not just to add capacity at the system's nominal operating voltage, but to increase the initial terminal voltage of the battery. For example, an application that requires 1 mA @5 V for five hours may only operate two hours between charges from a battery composed of four 1.2 V cells. Initially, the cells may provide 1.4 V/cell but, by the time they've discharged 30% of their capacity (70% of nominal capacity remaining) they may provide only 1.0 V/cell, which is inadequate for 5 V operation. Typical battery discharge curves and design considerations may be found in Paul Horowitz, Winfield Hill, The Art of Electronics, Cambridge University Press, New York, 1989, pages 917-932. The addition of another cell, or possibly two, could extend operation to the five hour goal, but the additional cells increase the initial cost and weight of the system.
The system design is further complicated by the fact that the load may vary widely during operation. A printer or radio transmitter operating from the same battery as a laptop computer may impose significant but fleeting additional loads upon the battery. The increased load current causes an increased voltage drop across the voltage regulator's pass transistor. If, for example, the regulator's PNP pass transistor exhibits 30.OMEGA. collector resistance and at 1 mA the headroom required for regulation is 30 mV, at 100 mA the headroom requirement would be 3 V.
A system designer could include enough cells in the system to accommodate worst-case load current over the system's nominal period of operation, but this approach may unnecessarily burden the design (in terms of weight, cost and volume). A controller (microprocessor or other control circuit) could use an alarm signal which indicates that the regulator's input voltage is approaching the limit for proper regulation to initiate power-saving measures such as power-cycling, or other measures to extend the system's operation. For example, an alarm signal may prompt a laptop computer to dim its LCD backlight or to disable printing or transmitting data via an attached cellular modem, while also alerting the laptop user to the fact that the batteries must be re-charged. Such low-power design techniques are discussed in Paul Horowitz, Winfield Hill, The Art of Electronics, Cambridge University Press, New York, 1989, pages 938-940.
If the above-mentioned alarm is based solely upon the unregulated input voltage to the voltage regulator the alarm may be triggered when, in fact, a great deal of capacity remains in the batteries. If the alarm were base upon both the voltage across the pass transistor and the load current, the controller could make a "more reasoned" determination of when to reduce operations (and load current), whether through power-cycling, disabling some functions, powering down after storing vital information.
For the forgoing reasons, there is a need for a voltage regulator which provides an indication that the input voltage to the regulator is too low to provide a regulated output voltage for the load attached to the regulator at the time.